Geochemistry and Petrogenesis of Northwest Africa 10401: A New Type of the Mg‐Suite Rocks

Abstract

AbstractThe petrogenetic models of the lunar crust are built on the returned Apollo and Luna samples collected from limited parts of the lunar nearside that are chemically unusual (i.e., material rich in K, Rare Earth Elements, and P [KREEP]) and not representative of the entire lunar lithologic suite. The lunar Mg‐suite is part of this sample collection and ubiquitously has geochemical characteristics indicating the involvement of KREEP in their petrogenesis and seemed to be linked to the Procellarum KREEP Terrain (PKT). However, it is unclear if KREEP is necessary for Mg‐suite magmatism or whether Mg‐suite magmatism was a global event that occurred without significant KREEP contribution, and thus, Mg‐suite rocks outside of the PKT region may exist without containing a significant KREEP signature. Here, we investigate lunar meteorite Northwest Africa (NWA) 10401, an anorthositic troctolitic breccia with a granulitic texture. NWA 10401 shares many characteristics of Apollo Mg‐suite rocks: both its bulk rock composition and alumina content, as well as its mineralogy and mineral chemistry, are more consistent with typical Apollo Mg‐suite rocks, rather than ferroan anorthosites. In addition, olivine‐spinel equilibria calculations indicate that NWA 10401 is consistent with being derived from a common parent to the Apollo Mg‐suite troctolites. However, despite these many shared characteristics, NWA 10401 is strongly depleted in REE, starkly separating it from the typical Apollo Mg‐suite of the PKT. This indicates that NWA 10401 (and pairs) could represent a Mg‐suite component outside the PKT, and thus, KREEP‐poor Mg‐suite magmatism may have been a global phenomenon on the Moon.